1. Field of the Invention
The present invention relates to an image forming apparatus such as a display apparatus using an electron beam and, more particularly, to an image forming apparatus having a support member (spacer) inside the envelope of the image forming apparatus.
2. Description of the Related Art
Conventionally, two types of devices, namely hot and cold cathode devices, are known as electron-emitting devices. Known examples of the cold cathode devices are surface-conduction emission (SCE) type electron-emitting devices, field emission type electron-emitting devices (to be referred to as FE type electron-emitting devices hereinafter), and metal/insulator/metal type electron-emitting devices (to be referred to as MIM type electron-emitting devices hereinafter).
A known example of the surface-conduction emission type electron-emitting devices is described in, e.g., M. I. Elinson, xe2x80x9cRadio Eng. Electron Phys., 10, 1290 (1965) and other examples will be described later.
The surface-conduction emission type electron-emitting device utilizes the phenomenon that electrons are emitted from a small-area thin film formed on a substrate by flowing a current parallel through the film surface. The surface-conduction emission type electron-emitting device includes electron-emitting devices using an Au thin film [G. Dittmer, xe2x80x9cThin Solid Filmsxe2x80x9d, 9,317 (1972)], an In2O3 SnO2 thin film [M. Hartwell and C. G. Fonstad, xe2x80x9cIEEE Trans. ED Conf.xe2x80x9d, 519 (1975)], a carbon thin film [Hisashi Araki et al., xe2x80x9cVacuumxe2x80x9d, Vol. 26, No. 1, p. 22 (1983)], and the like, in addition to an SnO2 thin film according to Elinson mentioned above.
FIG. 20 is a plan view showing the surface-conduction emission type electron-emitting device by M. Hartwell et al. described above as a typical example of the device structures of these surface-conduction emission type electron-emitting devices. Referring to FIG. 20, numeral 3001 denotes a substrate; and 3004, a conductive thin film made of a metal oxide formed by sputtering. This conductive thin film 3004 has an H-shaped pattern, as shown in FIG. 20. An electron-emitting portion 3005 is formed by performing electrification processing (referred to as forming processing to be described later) with respect to the conductive thin film 3004. An interval L in FIG. 20 is set to 0.5 to 1 mm, and a width W is set to 0.1 mm. The electron-emitting portion 3005 is shown in FIG. 20 in a rectangular shape at almost the center of the conductive thin film 3004 for the sake of illustrative convenience. However, this does not exactly show the actual position and shape of the electron-emitting portion 3005.
In the above surface-conduction emission type electron-emitting devices by M. Hartwell et al. and the like, typically the electron-emitting portion 3005 is formed by performing electrification processing called forming processing for the conductive thin film 3004 before electron emission. That is, the forming processing is to form an electron-emitting portion by electrification. For example, a constant DC voltage or a DC voltage which increases at a very low rate of, e.g., 1 V/min is applied across the two ends of the conductive thin film 3004 to partially destroy or deform the conductive thin film 3004, thereby forming the electron-emitting portion 3005 with an electrically high resistance. Note that the destroyed or deformed part of the conductive thin film 3004 has a fissure. Upon application of an appropriate voltage to the conductive thin film 3004 after the forming processing, electrons are emitted near the fissure.
Known examples of the FE type electron-emitting devices are described in W. P. Dyke and W. W. Dolan, xe2x80x9cField emissionxe2x80x9d, Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, xe2x80x9cPhysical properties of thin-film field emission cathodes with molybdenium conesxe2x80x9d, J. Appl. Phys., 47, 5248 (1976).
FIG. 21 is a cross-sectional view showing a typical example of the FE type device structure (device by C. A. Spindt et al. described above). Referring to FIG. 21, numeral 3010 denotes a substrate; 3011, an emitter wiring layer made of a conductive material; 3012, an emitter cone; 3013, an insulating layer; and 3014, a gate electrode. In this device, a voltage is applied between the emitter cone 3012 and the gate electrode 3014 to emit electrons from the distal end portion of the emitter cone 3012.
As another FE type device structure, there is an example in which an emitter and a gate electrode are arranged on a substrate to be almost parallel to the surface of the substrate, in addition to the multilayered structure of FIG. 21.
A known example of the MIM type electron-emitting devices is described in C. A. Mead, xe2x80x9cOperation of Tunnel-Emission Devicesxe2x80x9d, J. Appl. Phys., 32,646 (1961). FIG. 22 shows a typical example of the MIM type device structure. FIG. 22 is a cross-sectional view of the MIM type electron-emitting device. Referring to FIG. 22, numeral 3020 denotes a substrate; 3021, a lower electrode made of a metal; 3022, a thin insulating layer having a thickness of about 100 A; and 3023, an upper electrode made of a metal and having a thickness of about 80 to 300 A. In the MIM type electron-emitting device, an appropriate voltage is applied between the upper electrode 3023 and the lower electrode 3021 to emit electrons from the surface of the upper electrode 3023.
Since the above-described cold cathode devices can emit electrons at a temperature lower than that for hot cathode devices, they do not require any heater. The cold cathode device therefore has a structure simpler than that of the hot cathode device and can be micropatterned. Even if a large number of devices are arranged on a substrate at a high density, problems such as heat fusion of the substrate hardly arise. In addition, the response speed of the cold cathode device is high, while the response speed of the hot cathode device is low because it operates upon heating by a heater.
For this reason, applications of the cold cathode devices have enthusiastically been studied.
Of cold cathode devices, the above surface-conduction emission type electron-emitting devices are advantageous because they have a simple structure and can be easily manufactured. For this reason, many devices can be formed on a wide area. As disclosed in Japanese Patent Laid-Open No. 64-31332 filed by the present applicant, a method of arranging and driving a lot of devices has been studied. Regarding applications of surface-conduction emission type electron-emitting devices to, e.g., image forming apparatuses such as an image display apparatus and an image recording apparatus, electron-beam sources, and the like have been studied.
As an application to image display apparatuses, in particular, as disclosed in the U.S. Pat. No. 5,066,833 and Japanese Patent Laid-Open Nos. 2-257551 and 4-28137 filed by the present applicant, an image display apparatus using the combination of an surface-conduction emission type electron-emitting device and a fluorescent substance which emits light upon reception of an electron beam has been studied. This type of image display apparatus using the combination of the surface-conduction emission type electron-emitting device and the fluorescent substance is expected to have more excellent characteristics than other conventional image display apparatuses. For example, in comparison with recent popular liquid crystal display apparatuses, the above display apparatus is superior in that it does not require a backlight because it is of a self-emission type and that it has a wide view angle.
A method of driving a plurality of FE type electron-emitting devices arranged side by side is disclosed in, e.g., U.S. Pat. No. 4,904,895 filed by the present applicant. As a known example of an application of FE type electron-emitting devices to an image display apparatus is a flat display apparatus reported by R. Meyeretal. [R. Meyer: xe2x80x9cRecent Development on Microtips Display at LETIxe2x80x9d, Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6-9 (1991)].
An example of an application of a larger number of MIM type electron-emitting devices arranged side by side to an image display apparatus is disclosed in Japanese Patent Laid-Open No. 3-55738 filed by the present applicant.
Of image display apparatuses using electron-emitting devices like the ones described above, a thin, flat display apparatus receives a great deal of attention as an alternative to a CRT (Cathode-Ray Tube) display apparatus because of a small space and light weight.
FIG. 23 is a perspective view of an example of a display panel for a flat image display apparatus where a portion of the panel is removed for showing the internal structure of the panel.
In FIG. 23, numeral 3115 denotes a rear plate; 3116, a side wall; and 3117, a face plate. The rear plate 3115, the side wall 3116, and the face plate 3117 form an envelope (airtight container) for maintaining the inside of the display panel vacuum.
The rear plate 3115 has a substrate 3111 fixed thereto, on which Nxc3x97M cold cathode devices 3112 are provided (M, N=positive integer equal to xe2x80x9c2xe2x80x9d or greater, appropriately set in accordance with an object number of display pixels). As shown in FIG. 23, the Nxc3x97M cold cathode devices 3112 are arranged with M row-direction wirings 3113 and N column-direction wirings 3114. The portion constituted with the substrate 3111, the cold cathode devices 3112, the row-direction wiring 3113, and the column-direction wiring 3114 will be referred to as xe2x80x9cmulti electron-beam sourcexe2x80x9d. At an intersection of the row-direction wiring 3113 and the column-direction wiring 3114, an insulating layer (not shown) is formed between the wirings, to maintain electrical insulation.
Further, a fluorescent film 3118 made of a fluorescent substance is formed under the face plate 3117. The fluorescent film 3118 is colored with red, green and blue, three primary color fluorescent substances (not shown). Black conductive material (not shown) is provided between the fluorescent substances constituting the fluorescent film 3118. Further, a metal back 3119 made of Al or the like is provided on the surface of the fluorescent film 3118 on the rear plate 3115 side.
In FIG. 23, symbolsDx1 to Dxm, Dy1 to Dyn, and Hv denote electric connection terminals for airtight structure provided for electrical connection of the display panel with an electric circuit (not shown). The terminals Dx1 to Dxm are electrically connected to the row-direction wiring 3113 of the multi electron-beam source; Dy1 to Dyn, to the column-direction wiring 3114; and Hv, to the metal back 3119.
The inside of the airtight container is exhausted at about 10xe2x88x926 Torr. As the display area of the image display apparatus becomes larger, the image display apparatus requires a means for preventing deformation or damage of the ear plate 3115 and the faceplate 3117 caused by a difference in pressure between the inside and outside of the airtight container. If the deformation or damage is prevented by heating the rear plate 3115 and the face plate 3117, not only the weight of the image display apparatus increases, but also image distortion and parallax are caused when the user views the image from an oblique direction. To the contrary, in FIG. 23, the display panel comprises a structure support member (called a spacer or rib) 3120 made of a relatively thin glass to resist the atmospheric pressure. With this structure, the interval between the substrate 3111 on which the multi beam-electron source is formed, and the face plate 3117 on which the fluorescent film 3118 is formed is normally kept at submillimeters to several millimeters. As described above, the inside of the airtight container is maintained at high vacuum.
In the image display apparatus using the above-described display panel, when a voltage is applied to the cold cathode devices 3112 via the outer terminals Dx1 to Dxm and Dy1 to Dyn, electrons are emitted by the cold cathode devices 3112. At the same time, a high voltage of several hundreds V to several kV is applied to the metal back 3119 via the outer terminal Hv to accelerate the emitted electrons and cause them collide with the inner surface of the face plate 3117. Consequently, the respective fluorescent substances constituting the fluorescent film 3118 are excited to emit light, thereby displaying an image.
The above-mentioned electron beam apparatus of the image forming apparatus or the like comprises an envelope for maintaining vacuum inside the apparatus, an electron source arranged inside the envelope, a target on which an electron beam emitted by the electron source is irradiated, an acceleration electrode for accelerating the electron beam toward the target, and the like. In addition to them, a support member (spacer) for supporting the envelope from its inside against the atmospheric pressure applied to the envelope is arranged inside the envelope.
The display panel of this image display apparatus suffers the following problem.
Some of electrons emitted near the spacer strike the spacer, or ions produced by the action of emitted electrons attach to the spacer. Further, some of electrons which have reached the face plate are reflected and scattered to strike the spacer to charge the spacer. The orbits of electrons emitted by the cold cathode devices are changed by the charge-up of the spacer, and the electrons reach positions different from proper positions on the fluorescent substances. As a result, a distorted image is displayed near the spacer.
It is an object of the present invention to solve the problems of the support member.
The first aspect of an image forming apparatus according to the present invention has the following arrangement.
An image forming apparatus comprising a rear substrate having an electron-emitting device, a front substrate having an image forming member, and a support member for maintaining an interval between the rear substrate and the front substrate,
is characterized in that the apparatus comprises an electrode for applying a deflection force in a direction away from the support member to an electron emitted by the electron-emitting device, the support member has insulating properties, and the electrode relaxes deflection of an electron emitted by the electron-emitting device toward the support member owing to the insulating properties of the support member. According to the present invention, since said support member has insulating properties, an electron emitted from said electron-emitting device is deflected toward said support member. In such a situation, the degree of the deflection toward the support member can be reduced by the deflection by providing said electrode with the support member in comparison with the degree of deflection without electrode. In other words, the distance between the position on the image forming member, to which the electron is irradiated, and the support member can controlled. The reduction of the degree of deflection can be controlled to the suitable degree by changing the length of the electrode.
The second aspect of the image forming apparatus according to the present invention has the following arrangement.
An image forming apparatus comprising a rear substrate having an electron-emitting device, a front substrate having an image forming member, and a support member for maintaining an interval between the rear substrate and the front substrate,
is characterized in that the apparatus comprises an electrode for applying a deflection force in a direction away from the support member to an electron emitted by the electron-emitting device, the support member maintains a state in which a charge-up amount is substantially constant, and the electrode relaxes deflection of an electron emitted by the electron-emitting device toward the support member owing to charge-up of the support member. According to the present invention, since said support member has been charged up, an electron emitted from said electron-emitting device is deflected toward said support member. In such a situation, the degree of the deflection toward the support member can be reduced by deflection by providing said electrode with the support member in comparison with the degree of deflection without electrode. In other words, the distance between the position on the image forming member, to which the electron is irradiated, and the support member can controlled. The reduction of the degree of deflection can be controlled to the suitable degree by changing the length of the electrode.
In the present invention, the degree of the insulating properties of the support member, or the degree of the characteristic of maintaining the state in which the charge-up amount is substantially constant is set large enough to maintain the state in which the charge-up of the support member is almost stable in driving the electron-emitting device. More specifically, in an arrangement wherein the electron-emitting device is periodically driven, the above characteristic is a characteristic capable of suppressing a change in charge-up amount within the allowable range of a change in degree of deflection of an electron emitted by the electron-emitting device upon a change in charge-up amount of the support member during at least one period. In the first or second aspect, the electrode for applying the deflection force in the direction away from the support member to an electron emitted by the electron-emitting device is arranged, e.g., on the support member or between the support member and the electron-emitting device.
The third aspect of the image forming apparatus according to the present invention has the following arrangement.
An image forming apparatus comprising a rear substrate having an electron-emitting device, a front substrate having an image forming member, and a support member for maintaining an interval between the rear substrate and the front substrate,
is characterized in that the support member has insulating properties, and comprises an electrode for applying a deflection force in a direction away from the support member to an electron emitted by the electron-emitting device.
The fourth aspect of the image forming apparatus according to the present invention has the following arrangement.
An image forming apparatus comprising a rear substrate having an electron-emitting device, a front substrate having an image forming member, and a support member for maintaining an interval between the rear substrate and the front substrate,
is characterized in that the support member maintains a state in which a charge-up amount is substantially constant, and comprises an electrode for applying a deflection force in a direction away from the support member to an electron emitted by the electron-emitting device.
In the third or fourth aspect, the electrode of the support member relaxes deflection of an electron emitted by the electron-emitting device toward the support member owing to charge-up of the support member. That is, since said support member has been charged up, an electron emitted from said electron-emitting device is deflected toward said support member. In such a situation, the degree of the deflection toward the support member can be reduced by deflection by providing said electrode with the support member in comparison with the degree of deflection without electrode. In other words, the distance between the position on the image forming member, to which the electron is irradiated, and the support member can controlled. The reduction of the degree of deflection can be controlled to the suitable degree by changing the length of the electrode.
In the respective aspects described above, the electrode may be connected to wiring arranged on the rear substrate. In the respective aspects described above, a low potential is preferably applied to the electrode for deflecting the electron in the direction away from the support member. This electrode is desirably set at a low resistance in order to prevent the low potential of the electrode on the rear substrate from increasing toward the front substrate.
In the respective aspects described above, the electrode of the support member or the electrode arranged between the support member and the electron-emitting device is set at a low potential in order to allow the electrode to apply a force for deflecting an electron emitted by the electron-emitting device in the direction away from the support member. More specifically, the electrode enables the equipotential plane to have a normal line in the direction away from the support member.
In the respective aspects described above, the support member preferably comprises the electrode, and the electrode is preferably arranged on a portion of the support member near the rear substrate and is not arranged on a side near the front substrate over a predetermined position on the support member. A low potential is preferably applied to the electrode in order to deflect an electron in the direction away from the support member. The predetermined position is a position where the probability of discharge can be decreased without posing any practical problem because discharge may occur due to a potential difference between a high potential on or near the front substrate and the potential of the electrode. More specifically, when a distance between the rear substrate and the front substrate is 0.5 mm to 10 mm, a size of a pixel formed on the front substrate upon reception of an emitted electron is 100 xcexcm to 1 mm, and an accelerating voltage for accelerating an electron emitted by the electron-emitting device toward the image forming member is 1 to 15 kV, the predetermined position preferably corresponds to not more than xc2xc to not less than {fraction (1/20)} of the distance between the rear substrate and the front substrate.
In the respective aspects, the support member may comprise the electrode, and the electrode may be arranged to abut against the rear substrate. Particularly when the support member is arranged on wiring on the rear substrate, and the electrode is arranged to abut against the wiring, the support member can be satisfactorily connected to the rear substrate by arranging the electrode also on the abutment surface of the support member against the rear substrate.
In the respective aspects described above, the insulating properties of the support member or the characteristic of maintaining a state in which a charge-up amount is substantially saturated is given by a film formed on a surface of the support member. More specifically, the support member comprises a very-high-resistance film.
In the respective aspects described above, if the support member has a sheet resistance of not less than 1012 xcexa9/sq, the charged state of the support member can be kept almost stable.
In the respective aspects described above, the apparatus may further comprise a plurality of electron-emitting devices.
In the respective aspects described above, the apparatus may further comprise a plurality of electron-emitting devices arranged substantially linearly, and wherein the electrode causes deflection to such a degree as to set an interval between irradiation points, on the image forming member, of electrons emitted by, of the plurality of electron-emitting devices arranged substantially linearly, electron-emitting devices adjacent to each other via the support member to be almost equal to an interval between irradiation points, on the image forming member, of electrons emitted by electron-emitting devices adjacent to each other without mediacy of the support member. This arrangement is particularly suitable for suppressing distortion of an image to be formed.
In the respective aspects described above, the shape of the electrode can be selected from various shapes such as a layered shape and a block shape.
In the respective aspects described above, an acceleration electrode for applying a voltage for accelerating an electron emitted by the electron-emitting device toward the image forming member may be arranged. The acceleration electrode may be arranged on, e.g., the front substrate.
The principles of the present invention according to the present specification will be explained with reference to FIG. 1. FIG. 1 is a schematic cross-sectional view showing the basic structure of an image forming apparatus according to the present invention which is cut out along the line Axe2x80x94Axe2x80x2 in FIG. 16. Numeral 31 denotes a rear plate including an electron source substrate; 30, a face plate including fluorescent substances and a metal back; 20, a spacer; 21, an electrode formed of a low-resistance film; 13, wiring; 25, an equipotential line; 111, a device; and 112, an electron beam orbit.
In this structure, the spacer is charged by an electron emitted by a device 111 near the spacer 20. This charge-up is saturated after a while upon the start of driving. The amount of charge-up is constant. In this case, the electron emitted by the device near the spacer travels in the direction to move apart from the spacer near the rear plate due to the presence of the electric field (like the one indicated by the equipotential lines 25) generated by the electrode 21. Then, the electron travels in the direction to come close to the spacer due to the presence of the electric field (like the one represented by the equipotential lines 25) generated by the charge-up near the face plate. As a result, the electron can reach a proper position to obtain an image free from distortion. Since no current flows through the spacer, the charges of the spacer are eliminated slowly. For example, the charges cannot be eliminated at about 60 Hz as a scanning interval for an NTSC image, and the potential distribution of the space is kept unchanged. Therefore, the electron always reaches the same position regardless of the electron emission amount, and thus an image free from fluctuation can be obtained.
The low-resistance electrode 21 (to be referred to as an intermediate layer hereinafter) of the spacer may extend to the abutment surface of the spacer against the electron source substrate, as shown in FIG. 2. In this case, the conductive state between the electron source substrate and the low-resistance electrode (intermediate layer) on the side surface of the spacer in contact with the electron source substrate can be preferably improved.
An insulating film 22 may be formed on the surface of the insulating member 20 serving as the spacer of the present invention, as shown in FIG. 3. If the secondary electron-emitting efficiency of the insulating film is lower than that of the spacer substrate, the charge-up amount becomes smaller than that in the case not using any insulating film. The electrode on the rear plate side can be suppressed low to increase the discharge breakdown voltage.
As shown in FIG. 4, another electrode (intermediate layer) for setting the spacer at the same potential as that of the face plate may be formed on the abutment surface of the spacer of the present invention against the face plate and the side surface of the spacer in contact with the face plate in order to suppress discharge at a small gap between the face plate and the spacer.
The image forming apparatus of the present invention has the following forms.
{circle around (1)} The cold cathode device is a cold cathode device having a conductive film including an electron-emitting portion between a pair of electrodes, and preferably a surface-conduction emission type electron-emitting device.
{circle around (2)} The electron source is an electron source having a simple matrix layout in which a plurality of cold cathode devices are wired in a matrix by a plurality of row-direction wirings and a plurality of column-direction wirings.
{circle around (3)} The electron source is an electron source having a ladder-shaped layout in which a plurality of rows (to be referred to as a row direction hereinafter) of a plurality of cold cathode devices arranged parallel and connected at two terminals of each device are arranged, and a control electrode (to be referred to as a grid hereinafter) arranged above the cold cathode devices along the direction (to be referred to as a column direction hereinafter) perpendicular to this wiring controls electrons emitted by the cold cathode devices.
{circle around (4)} According to the concepts of the present invention, the present invention is not limited to an image forming apparatus suitable for display. The above-mentioned image forming apparatus can also be used as a light-emitting source instead of a light-emitting diode for an optical printer made up of a photosensitive drum, the light-emitting diode, and the like. At this time, by properly selecting m row-direction wirings and n column-direction wirings, the image forming apparatus can be applied as not only a linear light-emitting source but also a two-dimensional light-emitting source. In this case, the image forming member is not limited to a substance which directly emits light, such as a fluorescent substance used in embodiments (to be described below), but may be a member on which a latent image is formed by charging of electrons.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.